1. Field of the Invention
This invention relates to an optical sensor.
2. Description of the Related Art
For example, Jpn. Pat. Apply. KOKAI Publication 57-141604 discloses an optical fiber. As shown in FIG. 5A, the optical fiber 101 has a core 101a, a cladding 101b covering the core 101a, and a light absorbing part 101c which is provided in the cladding 101b. 
How light propagates in the optical fiber 101 will be explained.
As shown in FIG. 5A, in the case, the optical fiber 101 extends straight, light 103a propagating in the axial direction of the optical fiber 101 is guided at all. Light 103b propagating at a first angle to the axial direction is absorbed by the light absorbing part 101c. Light 103c propagating at a second angle to the axial direction is not absorbed by the light absorbing part 101c, is totally reflected by the cladding 101b and is guided forwards.
As shown in FIG. 5B, the optical fiber 101 may be bent at the light absorbing part 101c. In this case, lights 103, 103b and 103c propagate toward the light absorbing part 101c. As a result, the lights 103, 103b and 103c are absorbed by the light absorbing part 101c, and will no longer propagate.
The amount of light guided is thus controlled.
The optical fiber 101 so configured may be used in such a curvature measuring device 110 as shown in FIG. 5C. The device is a representative optical sensor which measures amount of displacement of the optical fiber 101. The curvature measuring device 110 shown in FIG. 5C has an optical fiber 101 shown in FIG. 5A and laid along a rail 111, a laser light source 113 connected to one end part of the optical fiber 101, and a photoelectric transducer device 115 connected to the other end part of the optical fiber 101. The optical fiber 101 is bent in compliance with the curvature of the rail 111. In proportion to the curvature of the rail 111, the light decreases in amount as it propagates in the optical fiber 101 from the laser light source 113 to the photoelectric transducer device 115. The photoelectric transducer device 115 measures the decrease in the amount of light. From the decrease in the amount of light, the curvature of the rail 111 and the downward flexure of the rail 111 will be determined when a train passes through.
As shown in FIG. 5C, for the optical sensor is arranged in the outdoor, the optical sensor cannot easily be incorporated in small high-precision devices in view of the arrangement of the optical fiber 101. In the optical sensor shown in FIG. 5C, the laser light source 113 is arranged at one end part of the optical fiber 101, and the photoelectric transducer device 115 is arranged at the other end part of the optical fiber 101. The optical sensor shown in FIG. 5C is inevitably large.
In view of the above, this invention has been made to provide a small optical sensor that can be incorporated in small high-precision devices.